The invention is generally directed to the field of sand blasting specific patterns by use of a preformed maskant and more particularly, but not by way of limitation, to the grit drilling of perforated patterns having a constant hole and spacing pattern in advanced composite materials used for aircraft acoustic structures having compound contoured surfaces.
The state of the art in sound attenuation structures for aircraft engine nacelles is represented by U.S. Pat. No. 4,254,171. This patent discloses a broad band noise attenuation panel that comprises an imperforate backing sheet, a cellular core adhesively bonded to the said backing sheet and to a perforate face sheet having a plurality of small perforations arranged in a predetermined spacing pattern. A microporous wire cloth is then adhesively bonded to the perforate sheet. Since this sound attenuation structure is used in connection with aircraft engine nacelles, it is made in arcuate and compound curved shapes to accommodate the dimensions and lines of the engine inlet and fan duct surfaces.
In the structure of U.S. Pat. No. 4,254,171, the elements would characteristically be made of metal with the perforated face sheet being typically made from aluminum. In view of the stress being placed in modern day aircraft design to reduce weight it is desired to replace as many of the elements of the subject acoustic structure with advanced composite materials thereby reducing weight without reducing strength.
Prior to the present invention, attempts at making a perforated sheet from cured imperforate advanced composite materials had been unsuccessful. As opposed to applications which involve cutting large artistic designs in an imperforate sheet the present application is unique in that it involves providing relatively small apertures, on the order of 0.030 to 0.125 inch, diameter typically arranged in a square or triangular pattern having a predetermined spacing so as to then provide a percent of open area (POA) to satisfy structural and/or acoustic requirements.
Adding to the difficulty of providing the required advanced composite perforate sheet is the fact that advanced composite sheets are relatively soft in an uncured condition and are relatively hard in a cured state and are cured into a desired arcuate or compound contoured shape by the application of required heat and pressure while the sheet is held in appropriate tooling within a vacuum bag that is positioned within a suitable autoclave.
Drilling the perforations in a cured advanced composite sheet was tried but was unsuccessful. In addition to having to deal with a curved surface which made the procedure very expensive to pursue, it was found that drilling can cause microfractures and scouring in the material at the hole sidewall and thus reduce the strength of the material to an unacceptable level.
Pin molding of the perforations directly into the composite material has been successfully accomplished for relatively large holes with low POA and flat or gently contoured surfaces. However, tooling costs can be prohibitive and material penetration/removal difficult for large compound contour parts with smaller holes and higher POA.
Laser drilling was also tried but was undesirable due to crazing microcracking at the hole sidewall caused by the heat affected zone radiating from the drilled holes. Attempts to provide the requisite apertures by a punching operation were equally unsuccessful since the punching operation was found to induce microfractures and tearing of composite fiber bundles resulting in an unacceptable reduction in material strength.
Thus, until the provision of the present invention no method had been discovered which would satisfactorily provide the required perforation pattern in relatively large parts at reasonable cost without degrading other characteristics of the material to an unacceptable level.
The prior art teaches the use of stencils with sandblasting for producing duplicates which are typically flowers or other artistic or fanciful designs, or written or printed material or designs as applicable to the marking of gravestones, decorative wall murals, commercial business signs and the like. There is no teaching regarding gas propelled abrasive blasting to provide through cutting of resin matrix based composite materials, such as graphite fiber/epoxy resin, graphite fiber/polyimide resin, glass fiber/epoxy resin, carbon/carbon and the like where the resulting perforated material is used for light-weight structural or structural and acoustic applications. The prior art focuses on flat surfaces whereas the present invention includes complex compound contour surfaces.
In U.S. Pat. No. 767,362, issued to E. C. Phillips in Aug. 9, 1904, titled "Process of Perforating Music Sheets" is a typical example of the use of a stencil as a guide to perforate limp music sheets (player piano and the like rolls). Thin, limp, easily cut material, (paper or the like) typically in roll form of constant width, is perforated by passing the sheet, covered with a nonadhering annular pattern web, over a flanged rotating cylindrical impact drum or roller using mechanical devices (analogous to a motion picture projector film tension roller mechanism) to retain the pattern web and pliable material in contact against each other and pressed against the impact drum. While passing over the drum, the pattern web is blasted with an air and sand mixture to perforate the limp material. The reference teaches a fan type nozzle for emitting the air and sand in a wide but narrow pattern to cover the width of the music sheet which results in the abrasive media impacting the pliable material at different angles, approximately 90.degree. to the material surface directly above the center line of the nozzle and approaching 60.degree. to the material surface at the outer edge of the nozzle. With a thin limp soft material, such as paper or the like, the varying angle of abrasive impact will have no effect on the shape of the perforation through a thin material. However, with thicker material, such as an advanced composite material formed from cured graphite fibers, polyimide resin or epoxy resin and the like, the fan type nozzle and resulting abrasive impact angles will provide straight through perforations only directly below the nozzle center line. At the fan edges, the perforation side walls will be perforated at the same angle as the abrasive impact angles producing angled perforations which are tapered and elliptical rather than round as provided directly beneath the nozzle centerline. In addition the teaching is not suitable for relatively rigid large flat or contoured shapes of varying size.
The U.S. Pat. No. 2,358,710 issued to Harold R. Helgeson on Sept. 19, 1944, titled "Sandblast Stencil and Method of Making Same", teaches that it is known to clamp a reinforced stencil on a workpiece for sandblasting the stencil design upon work surfaces and the like. This teaching is limited to sandblasting artistic designs onto the surface of the typically flat workpiece. There are no teachings for the formation of perforations through the workpiece by this method.
In a similar manner U.S. Pat. No. 2,791,289 issued May 7, 1957 entitled "Process of Forming Fissured Fiber Acoustical Tile and Products Thereof" teaches the use of stencil and sandblasting to form a random pattern of irregular shapes into (but specifically not through) low density, flat, fibrous, acoustical tile for non-structural acoustic treatment of residential and commercial buildings.
S. S. White Industrial Products Div., of Pennwalt Corporation, manufactures and distributes abrasive jet machining equipment suited to cutting, drilling, marking, and etching of glassware or the like. This equipment utilizes small nozzles in the orifice size range of from 0.005" to 0.045". Nozzles of this configuration are used as described to typically produce fanciful designs on glass and clean deburr or trim, small devices such as threaded fasteners, electrical components and the like. This type of teaching is limited to small nozzles and/or very fine abrasive, typically 10-50 microns in size, with low abrasive feed rates typically less than 50 grams per minutes which can not produce an acceptable industrial cutting speed for producing a multitude of holes in large parts.
Thus, there has not been an entirely satisfactory method known, either in the aerospace industry or in the art of stencilling for perforating composite materials and the like having a thickness of approximately 0.015" or greater until the emergence of the instant invention.